1. Field
The present work relates generally to the wireless communications and, more particularly, to detecting the presence/absence of incumbent signals in television (TV) channels.
2. Background
The Federal Communications Commissions (FCC) is developing rules to allow low-power white space devices (WSD) to use unoccupied TV channels, referred to as TV white space, provided that WSDs do not cause harmful interference to TV receivers. Cognitive radio thus demands a technology that can periodically sense the radio frequency (RF) environment, dynamically identify unused spectral segments, and then operate in these white spaces without causing harmful interference to the incumbent users. As one of the essential functionalities of a WSD, spectrum sensing needs to reliably detect weak incumbent signals at very low signal-to-noise ratio (SNR). Incumbents include ATSC and NTSC TV broadcasts and Part 74 wireless microphones.
There are several prior art detection techniques for spectrum sensing. Some of the known detection techniques focus on extracting features from the power spectral density (PSD) estimate. If an incumbent signal exists at a certain frequency, the PSD estimate exhibits a value noticeably higher than the noise floor. Therefore from comparing the PSD estimate with known patterns associated with incumbent signals, the existence or absence of incumbent signals can be detected.
However, in a realistic receiver, even without an incumbent signal, there exist residual signals from adjacent channels, internal emissions, interference from unknown sources, RF impairments such as harmonic images and intermodulation distortions due to hardware constraints. In the PSD estimate, these non-ideal effects introduce spikes that are easily detected, and may then be improperly identified as incumbent signals, leading to false alarms. Unless these spurious spikes are identified as such, almost all the white spaces will be classified as occupied, thus largely preventing the desired operation of WSDs. FIG. 1 illustrates the PSD estimate of a typical white space TV channel in the absence of incumbent signaling.
The problem of improperly classifying spurious spikes as real incumbent signals becomes especially challenging for wireless microphone sensing. Part 74 wireless microphone signals are typically frequency modulated (FM) with a bandwidth no greater than 200 kHz, and their carrier frequencies can choose from a large number of possible locations within a TV channel. Furthermore, their features like tone keys are weakened when the signal is modulated by voice or music, and are completely eliminated by noise at low SNR. The wireless microphone signals therefore become essentially indistinguishable from spurious spikes in the TV channel spectrum.
It is desirable in view of the foregoing to provide for rejection of spurious interference in the detection of incumbent TV channel users, for example, wireless microphone users.